Problem: If $\angle LON$ is a right angle and $m \angle LOM = 80^\circ$, what is $m \angle MON$, in degrees? 80^\circ O L N M
Solution: From the diagram, we see that $\angle LOM$ and $\angle MON$ are complementary angles. Therefore, $m \angle LOM + m \angle MON = 90^\circ$ Thus, $m \angle MON = 90^\circ - m \angle LOM = 90^\circ - 80^\circ = 10^\circ$.